This is not scientific but I took an old Xeon V4 workstation and an old Quadro GPU and compared the Whisper times for the same approximately 4 minute audio, transcribing in English.
Here are the times which is slightly surpising.
I used the small model with this command:
whisper vid.mp4 --model small --output_format srt --language en
Of course to test the GPU I added the --device cuda flag and confirmed GPU was being used as utilization on GPU was 100%
real 3m49.033s
real 3m55.140s
I observed it took some time to load the model into GPU memory, for shorter audio this increased the overall processing time slightly. I believe if I have time to test a longer audio that the result will be different and GPU will show a better result.